A vehicle's engine typically utilizes an idle speed control mode where engine speed is controlled to a desired speed when a vehicle is stationary or slowly moving and an operator is not requesting drive torque. During idle conditions, it is desirable to maintain a constant engine speed, thereby giving the operator superior drive feel. To keep engine speed constant, idle speed control should reject engine torque disturbances from various sources, such as, for example, air conditioning systems, power steering systems, changes in ambient conditions, or changes in any other devices that affect engine speed.
One method for control ling engine speed to a desired speed uses ignition timing, throttle position, or a combination of both. In one system a torque reserve is used so that it is possible to rapidly increase engine torque using ignition timing, thereby controlling engine speed. One example of a system using ignition timing is disclosed in U.S. Pat. No. 5,765,527.
The inventors herein have recognized several disadvantages with the above approaches. In particular a disadvantage with using throttle position is that the throttle cannot quickly change engine torque since it controls flow entering an intake manifold. Controlling flow entering the manifold cannot rapidly control cylinder charge due to manifold volume. For example, if the throttle is instantly closed, cylinder air charge does not instantly decrease to zero. The engine must pump down the air stored in the manifold, which takes a certain number of revolutions. Therefore, the cylinder air charge gradually decreases toward zero.
Another disadvantage with the known approaches is related to ignition timing. In particular, to maximize fuel economy, ignition timing should be at MBT timing (ignition timing for maximum torque). However, when at MBT, adjustment of ignition timing in any direction decreases engine torque and fuel economy. Therefore, when maximizing fuel economy, load torques cannot be rejected since ignition timing can only decrease engine torque. To be able to use ignition timing in both positive and negative directions, ignition timing must be set away from MBT timing. This allows rapid engine torque control, but at the cost of degraded fuel economy.